1. Field of the Invention
The present invention relates to a combustion apparatus which is provided with a burner, a combustion box containing therein a heat exchanger which is heated by combustion gas from the burner, an exhaust passage in fluid communication with the combustion box, and a fan for supplying the burner with combustion air.
2. Description of the Related Art
This kind of combustion apparatus is likely to give rise to low-frequency resonant sounds below 100 Hz. This is due to the fact that an open pipe equivalent length (the length corresponding to an open pipe whose both ends are free ends) from a suction opening formed in a fan casing which houses therein fan impellers to an exhaust port on a downstream end of the exhaust passage becomes the length that corresponds to a multiple of half wavelength of frequency components below 100 Hz that are contained in the combustion noises generated by combustion vibrations, thereby giving rise to acoustic vibrations of these frequencies.
As a solution to this kind of problems, there is known a prior art in which is provided a predetermined length of air suction duct in fluid communication with the suction opening in the fan casing (e.g., see JP-UM-A-1989-129556). According to this arrangement, the substantial suction opening of the fan becomes the inlet on an upstream end of the air suction duct. The open pipe equivalent length from this inlet to the exhaust port thus becomes longer and deviates from a length corresponding to the multiple of half wavelength of the frequency components below 100 Hz contained in the combustion noises. As a result, the acoustic vibrations are suppressed and the low-frequency resonant sounds below 100 Hz can be suppressed. However, if the air suction duct is provided, there will be generated low-frequency resonant sounds in the range of 400 Hz to 800 Hz that did not occur in the case in which the air suction duct is not provided. This is due to the fact that the open pipe equivalent length from the inlet on the upstream side of the air suction duct to the exhaust port becomes the length corresponding to a multiple of half wavelength in the range of 400 Hz to 800 Hz that are contained in the combustion noises, thereby giving rise to the acoustic vibrations of that frequency range.
As a solution to this problem, there is known a prior art in which an auxiliary suction opening is formed in the neighborhood of the downstream end of the air suction duct which is in fluid communication with the suction opening in the fan casing (see, e.g., JP-A-1990-29505). According to this arrangement, the energy pressure of the frequency components that should give rise to acoustic vibrations in the range of 400 Hz to 800 Hz partly escapes from the auxiliary suction opening, thereby suppressing the acoustic vibrations of this frequency range. As a result, different low-frequency resonant sounds below 100 Hz and in the range of 400 Hz to 800 Hz can be suppressed.
However, if the auxiliary suction opening is formed in the air suction duct in the above-mentioned manner, the air from the auxiliary suction opening is sucked in a manner to flow across the suction air flow in the air suction duct. The air from the auxiliary suction opening thus interferes with the suction air flow in the air suction duct. As a result, the flow resistance in the air suction duct becomes higher.